Electromagnetic induction devices such as power transformers and reactors may be provided with On-Load Tap Changers (OLTC) for enabling stepped voltage regulation of the electromagnetic induction device as a means for voltage compensation when the electromagnetic induction device is On-Load, i.e. connected to a transmission or distribution network.
An OLTC unit which comprises a tap changer mechanism can either be of in-tank type or on-tank type. If the OLTC unit is arranged inside the electromagnetic core housing, i.e. the transformer tank or reactor tank, it is of in-tank type. If the OLTC unit is mounted to the electromagnetic core housing, it is of on-tank type. For the latter type of OLTC unit, the tap changer mechanism is separated from the interior of the electromagnetic core housing by means of an insulation barrier. The insulation barrier comprises electrical connections and acts as an interface between windings inside the electromagnetic core housing and the tap changer mechanism in the OLTC unit. Moreover, the insulation barrier separates dielectric fluid in the electromagnetic core housing from dielectric fluid in the OLTC unit, preventing mixing of the dielectric fluids and thus reducing the risk of one dielectric fluid contaminating the other.
The tap changer mechanism in the OLTC unit and the electromagnetic induction device windings, i.e. the active parts of the electromagnetic induction device, are provided with electrical insulation. This electrical insulation form an electrical insulation system which increases the dielectric withstand strength of the electromagnetic induction device. Such an electrical insulation is typically made of cellulose-based material. The active parts are provided with the electrical insulation mainly by means of manual labour. When the electrical insulation has been assembled with the active parts, the entire assembly is subjected to a drying process to remove moisture to increase the dielectric withstand strength of the electrical insulation and thus increase the lifetime of the electromagnetic induction device. The drying process may involve subjecting the electrical insulation system to vacuum and heating, sometimes combined with vaporised kerosene. When the drying process has been finalised, the final assembly and sealing of the electromagnetic induction device is performed by means of manual labour.